The present invention relates to a catalyst which is used for wet oxidation treatment for wastewater containing organic compounds etc. for decomposing them and also, relates to a process for producing the catalyst and a process for treating wastewater wet oxidation under the presence of the catalyst.
There have been hitherto known methods for treating wastewater, such as a biochemical method called the activated-sludge method, and a wet oxidation treatment called the Zimmerman method.
The activated-sludge method requires a long time to decompose organic compounds and also, requires the wastewater to be dilute up to a concentration suitable for the growth of algae and bacteria, so that it has the defect that a large area is required to set treating facilities.
The Zimmerman method comprises treating wastewater in the presence of an oxygen gas at a high temperature and high pressure and decomposing organic compounds in the wastewater. In this method, there has been proposed a method which comprises a use of various kinds of oxidation catalysts in order to accelerate the reaction rate. The oxidation catalyst used here is a catalyst with a compound of a noble metal such as palladium, platinum or the like supported on a carrier such as alumina, silica, silica gel, active carbon or the like.
In general, it is rare that the chemical species included in wastewater to be treated are always the same. Thus, in many cases, a nitrogen-containing compound is included with a nitrogen-free organic compound.
However, wastewater including a nitrogen-containing compound such as an amine compound, an amide compound, an amino acid compound or the like is not treated with satisfactory efficiency by the above-mentioned methods.
Wastewater including an amine is usually treated by a cohesive treating method in which an anionic macromolecular cohesive agent is used. This method comprises gathering an amine by the anionic macromolecular cohesive agent and eliminating a formed precipitate (or sludge) from wastewater. Also, there has been attempted an adsorbent method which comprises bringing wastewater in contact with various kinds of adsorbents such as active carbon, activated clay, a silica gel, a complex oxide gel and the like, and adsorbing the amine to the adsorbent to eliminate it from the wastewater.
Since sludge formed by the cohesive treating method contains amines, it should not be disposed of without a following treatment. Because of this, a treatment to decompose the amines in the sludge becomes necessary. In addition, the macromolecular cohesive agent is expensive, so the cost for the treatment becomes high.
Concerning the adsorbent method, the elimination percentage of amines is unsatisfactory enough. Since the adsorbing power of an adsorbent easily decreases, there is a problem in durability of the adsorbent.
Since the wet oxidation process is unavoidable in the wastewater treatment, it will be convenient if an arrangement is made so as to decompose a nitrogen-containing compound included in the wastewater in the course of the wet oxidation process.
On the other hand, a treatment for wastewater including a sulfur-containing compound has been so far carried out by different methods depending upon the state and nature of the sulfur-containing compound. For example, in a case of wastewater including an organic sulfur compound, a biological treatment is generally carried out. However, in a case where a compound containing thiophene and the like is treated, which affects badly upon organisms in sludge, a biological treatment can not be applied and, accordingly, a combustion treatment for example is carried out.
Wastewater containing a sulfide such as sodium sulfide or the like (for example, wood digestion wastewater in paper- and pulp manufacture, wastewater from a coke oven in steel manufacture, wastewater after fiber-washing, wastewater from a plant of petroleum chemical products such as ethylene, BTX and the like, as well as wastewater from a coal gasification plant, a petroleum-refining plant, a rayon factory and a dyeing plant) has been mostly treated by a method which comprises adding iron chloride into wastewater to solidify sulfur ions, removing solid iron sulfide by solid-liquid separation, adjusting the pH of the separated solution, carrying out a biological treatment of the solution, and then discharging the treated wastewater. Also, wastewater containing a sulfite salt and thiosulfate salt (for example, wastewater from a wood kiln of pulp-making factory, wastewater from photograph-developing, wastewater from metal treatment as well as alkaline wastewater used to absorb sulfur dioxide and the like) is treated by a method which comprises subjecting wastewater to neutralization-precipitation treatment followed by biological treatment and then discharging the treated wastewater.
When wastewater containing a sulfur-containing compound is treated by either one or both of a biological treatment and a combustion treatment, there exist the following problems to be solved. In the biological treatment, it is necessary to adjust a wastewater source solution by diluting it with water so that organisms are not badly affected. Therefore, wastewater to be treated becomes a large amount and facilities for the biological treatment must be arranged on a large scale, so that there is a serious problem in the necessary cost and so forth.
Also, in the combustion treatment, when the amount of heat generated from wastewater is low, a supplementary fuel must be added and also, because a large amount of sulfur is usually included in the wastewater, a large amount of sulfur oxides are formed and, therefore, it is necessary to arrange a desulfurizer.
Next, when wastewater including a sulfur-containing compound such as sulfide is treated, if a method which comprises removing the sulfur-containing compound as iron sulfide by adding iron chloride is applied, sludge having iron sulfide as a main component is formed and also, this method is complicate as a treating procedure, because it consists of the following steps: injection of solution of chemicals, solid and liquid separation, pH control and biological treatment.
The organic halogeno compounds have various uses because of their stability. Since they are nonflammable and have great capability to degrease, they have been used in a large amount as a degreasing cleaner in metal, machinery and electronics industries as well as a cleaner for dry cleaning. On the other hand, the compounds have brought about problems on various fields. In general, since the organic halogeno compounds are difficult to decompose, they are seriously accumulating in the natural environment and, as a result, ground water pollution has emerged everywhere. Furthermore, some of the organic halogeno compounds have been found to be carcinogenic to humans and, thus, trichloroethylene, tetrachloroethylene, 1,1,1-trichloroethane and the like have been designated as regulated items of the water-pollution preventive law on 1989, because of concern about influence on the human health.
Concerning treatment of the organic halogeno compounds, various methods have been proposed or used, and if roughly classified, there are a degradation method and a nondegradation method. Concerning the degradation method, there are listed a packed tower stripping method, a volatilizing method by means of exposure to air or heating, and an adsorption method using active carbon or macromolecules. Concerning the volatilizing method, the operation itself is very simple and at a low cost, but the method comprises only evaporating organic halogeno compounds in a liquid phase or a solution and dispersing them in air and, therefore, basically it does not settle environmental pollution caused by organic halogeno compounds. Concerning the adsorption method, secondary processing such as a recovering process after adsorption and a process to treat the adsorbent becomes necessary.
Concerning the degradation method, there are an irradiation method, a microorganism degradation method, a redox method and so forth. The irradiation method, of which representative examples are a photodecomposition method using a semiconductor as a catalyst and a radiation method, is still on an experimental stage and is not adopted for practical use. The microorganism degradation method takes a long time for treatment and its efficiency in treatment is unstable and, therefore, there exist many problems for a practical use. Concerning the redox method, a method of using an oxidizing agent such as ozone, hydrogen peroxide or the like and a method of reductive degradation method using iron have been attempted.
However, in a case where an organic halogeno compound exists in a high concentration, a method of highly efficient treatment has not yet been invented either as a nondegradation method or as a degradation method. In the volatilizing method, a large amount of organic halogeno compounds are discharged into air and, therefore, the method is not a fundamental solution for the organic halogeno compounds to be treated. The adsorption method has a short break-through time in the case of high concentration of organic halogenic compounds, so it is not practical. Concerning the degradation method, highly effective decomposition has not yet been a practical one, and also, there exists a problem that harmful decomposition products are secondarily generated. In short, a practical and fundamental method to remove the organic halogeno compounds is not yet developed at a present stage.